More than one-third of all counties in the U.S. lower 48 states face higher risks of water shortages by mid-century as the result of global warming, the Natural Resources Defense Council (NRDC) said today.

The findings by consulting firm Tetra Tech for NRDC–a nonprofit that advocates for protection of natural resources, public health, and the environment–used publicly available water-use data across the United States and climate projections from a set of models used in recent Intergovernmental Panel on Climate Change (IPCC) work to evaluate withdrawals related to renewable water supply, NRDC said in a news release.

“The report finds that 14 states face an extreme or high risk to water sustainability, or are likely to see limitations on water availability as demand exceeds supply by 2050. These areas include parts of Arizona, Arkansas, California, Colorado, Florida, Idaho, Kansas, Mississippi, Montana, Nebraska, Nevada, New Mexico, Oklahoma, and Texas. In particular, in the Great Plains and Southwest United States, water sustainability is at extreme risk.

“The more than 400 counties identified as being at greatest risk in the report reflects a 14-times increase from previous estimates,” NRDC said.

County- and state-specific maps detailing the report findings (including a Google Earth map) may be found here and here.

Crops worth $105 billion at risk

While detailed modeling of climate change impacts on crop production was beyond the scope of the Tetra Tech analysis, NRDC said, the potential scale of disruption is reflected based on the value of the crops produced in the 1,100 at-risk counties. “In 2007 the value of the crops produced in the at-risk counties identified in the report exceeded $105 billion.”

A separate study compared the Tetra Tech data with county-level crop production data from the U.S. Department of Agriculture; state-specific fact sheets outlining the potential agricultural impacts may be found here.

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Sandra Postel, Freshwater Fellow of the National Geographic Society and director of the Global Water Policy Project, a water research group, told Reuters that evidence had been mounting for years about diminishing water sustainability and more public policy work was needed to address the issue.

“Preparations need to begin in earnest,” Postel told Reuters. “The climate models tell us we should begin to prepare for this. It is preparing for what sounds like an unthinkable situation but increasingly looks like an unthinkable situation that may happen.”

The Tetra Tech analysis shows climate change will take a serious toll on water supplies throughout the country in the coming decades, with over one out of three U.S. counties facing greater risks of water shortages, said Dan Lashof, director of the NRDC Climate Center.

“Water shortages can strangle economic development and agricultural production and affected communities. As a result, cities and states will bear real and significant costs if Congress fails to take the steps necessary to slow down and reverse the warming trend.

“Water management and climate change adaptation plans will be essential to lessen the impacts, but they cannot be expected to counter the effects of a warming climate.

“The only way to truly manage the risks exposed by this report is for Congress to pass meaningful legislation that cuts global warming pollution and allows the U.S. to exercise global leadership on the issue,” Lashof said.

Figure 1: Water use by sectors in 2005 (Source: Kenny et al., 2009). Courtesy of NRDC

Sujoy Roy, principal engineer and lead report author, Tetra Tech, said: “The goal of the analysis is to identify regions where potential stresses, and the need to do something about them, may be the greatest.

“We used publicly available data on current water withdrawals for different sectors of the economy, such as irrigation, cooling for power generation, and municipal supply, and estimated future demands using business-as-usual scenarios of growth. We then compared these future withdrawals to a measure of renewable water supply in 2050, based on a set of 16 global climate model projections of temperature and precipitation, to identify regions that may be stressed by water availability.

“These future stresses are related to changes in precipitation as well as the likelihood of increased demand in some regions.”

Water withdrawal will grow by 25 percent in many areas of the U.S. including the arid Arizona/New Mexico area, the populated areas in the South Atlantic region, Florida, the Mississippi River basin, and Washington DC and surrounding regions, NRDC’s statement said.

“In many places, water is already used in quantities that exceed supply.”

“Estimated water withdrawal as a percentage of available precipitation is generally less than 5 percent for the majority of the Eastern United States, and less than 30 percent for the majority of the Western United States. But in some arid regions (such as Texas, the Southwest, and California) and agricultural areas, water withdrawal is greater than 100 percent of the available precipitation. In other words, in many places, water is already used in quantities that exceed supply.”

Tetra Tech projected future water demand and supply. Demand was projected based upon a business-as-usual scenario of continued population growth and associated energy and cooling water needs.

Supply–or available precipitation–was estimated from current and future temperature and precipitation scenarios, obtained from an ensemble of 16 global climate models. The analysis then compared future demand to future supply to provide an initial assessment of water resources sustainability across the nation–resolved at the county level, the best resolution for water use information–and helped identify areas most likely to be adversely impacted by increasing water demand and climate change.

Water supply sustainability index

The Tetra Tech report developed a new water supply sustainability index.

The risk to water sustainability is based on the following criteria:

Projected water demand as a share of available precipitation;

Groundwater use as a share of projected available precipitation;

Susceptibility to drought;

Projected increase in freshwater withdrawals; and

Projected increase in summer water deficit.

More detailed explanations of the study methodology and water sustainability criteria can be found here.

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